The thread-cutting process usually takes place as follows in a sewing machine equipped with a thread-cutting device: The sewing machine is stopped at the end of the seam with the needle in the lower position. The sewing machine is then driven briefly once again, in the course of which the arm shaft performs half a revolution and the hook a full revolution, and the take-up lever assumes the top dead center of its path of movement. In the course of this half revolution of the arm shaft, the catch thread device of the thread-cutting device performs a catching or separating movement, by which the threads to be cut are caught, during the time at which the hook has widened the needle thread loop. The cutting of the threads proper takes place only at the end of the half revolution of the arm shaft, when the take-up lever is stopped at the top dead center of its path of movement.
Since no sewing stitch formed by the looping of the hook and needle threads is formed by the thread cutting, the fabric-side needle thread end therefore emerges from the fabric after the thread cutting at the site of the last insertion of the needle, while the hook thread end hangs down from the fabric at the site of the last complete stitch. Since these two points are spaced apart from one another by one stitch length, i.e., the length of one feed step, the hook thread end is consequently longer than the needle thread end by the amount of the stitch length.
This situation and the result linked with it, which may have an adverse effect under certain circumstances, are shown in FIGS. 4 and 5 of the drawing in the case of a sewing machine with lower feed and in FIGS. 9 and 10 in the case of a sewing machine with combined lower feed and needle feed.
To avoid this situation, it is proposed in a thread-cutting device known from DE 16 85 087 B2 that the last stitch formation process, during which the thread cutting is to take place, be performed at the point at which the last complete sewing stitch was formed. Even though both the hook thread end and the needle thread end will thus hang down from the fabric at the same point and thus have the same length, this is achieved by accepting the drawback that a complete stitch must first be formed at this point after stopping the sewing machine and switching over the stitch length mechanism to zero stitch length before the half revolution of the arm shaft can take place as in the state of the art for performing the thread-cutting process. An additional stitch formation process must therefore be carried out in the prior-art thread-cutting device between the stopping of the machine and the thread cutting compared with the above-described state of the art. However, such an additional stitch formation process may lead overall to an undesired, considerable loss of time in the case of the sewing of a plurality of short seams.